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Physiological basis for human autonomic rhythms.

D L Eckberg1

  • 1Department of Medicine, Medical College of Virginia at Virginia Commonwealth University, and Hunter Holmes McGuire Department of Veteran Affairs Medical Center, Richmond, USA. deckberg@aol.com

Annals of Medicine
|August 19, 2000
PubMed
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Respiratory influences on heart rate and sympathetic nerve activity are complex. Current models for assessing sympathovagal balance using heart rate variability may be limited, especially when extrapolating from upright tilt measurements.

Area of Science:

  • Human neurophysiology
  • Autonomic nervous system regulation
  • Cardiovascular research

Background:

  • Oscillations in arterial pressure, heart period, and muscle sympathetic nerve activity (MSNA) are key to understanding human neurophysiology.
  • Respiratory rhythms are the most studied, with evidence suggesting a central 'gate' mechanism influences MSNA and R-R intervals.
  • Traditional views attribute parallel oscillations to baroreflexes, but emerging evidence points to direct respiratory modulation of cardiac autonomic motoneurones.

Purpose of the Study:

  • To critically evaluate the use of mathematical models, particularly low- and high-frequency R-R interval fluctuations, for characterizing sympathovagal balance.
  • To assess the validity of extrapolating findings from upright tilt measurements to supine resting conditions.
  • To clarify the mechanisms underlying respiratory influences on cardiovascular and sympathetic activity.
Keywords:
NASA Discipline CardiopulmonaryNon-NASA Center

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Main Methods:

  • Analysis of respiratory fluctuations in arterial pressures, heart periods (R-R intervals), and muscle sympathetic nerve activity.
  • Review of mathematical modeling approaches for heart rate variability (HRV) analysis, focusing on low-frequency (LF) and high-frequency (HF) components.
  • Examination of data from upright tilt studies and comparison with supine resting conditions.

Main Results:

  • The ratio of LF to HF R-R interval oscillations during upright tilt primarily reflects vagal (HF) withdrawal, not sympathetic (LF) increases.
  • The sympathetic component of HRV does not significantly change during tilt, challenging the interpretation of LF/HF ratio as a direct measure of sympathetic activity.
  • The validity of applying HRV measures from upright tilt to supine rest remains unestablished.

Conclusions:

  • Current mathematical models for assessing sympathovagal balance using HRV, particularly the LF/HF ratio, may be misinterpreted, especially when derived from upright tilt data.
  • Further research is needed to validate HRV measurement extrapolation between different body positions and to understand its prognostic significance in cardiovascular diseases.
  • It remains unclear if reduced HRV is a marker of disease severity or indicative of functional reflex abnormalities contributing to arrhythmias.